Thermal processing systems are widely used in various stages of semiconductor fabrication. Basic thermal processing applications include chemical deposition, diffusion, oxidation, annealing, silicidation, nitridation, and solder re-flow processes. Vertical rapid thermal processing (RTP) systems comprise a vertically oriented processing chamber which is heated by a heat source such as a resistive heating element or a bank of high intensity light sources. An elevator structure is controlled to move a wafer on a wafer support vertically within the processing chamber. In some RTP systems, the heat sources create a temperature gradient within the processing chamber and temperature ramp-up and ramp-down rates of the wafer being processed are controlled by the vertical location of the wafer within the processing chamber. Therefore, to optimize the thermal processing of semiconductor wafers it is important to accurately control the position of the wafer within the processing chamber.
Because of the length of the moving elevator structure it must be held rigidly at its base in order to obtain maximum mechanical rigidity, thereby minimizing in plane vibration that may be imparted to the wafer. The vibrations may cause the wafer to move relative to the support, thereby losing concentricity with the support structure. Lack of concentricity results in temperature non-uniformity near the wafer perimeter.
Semiconductor thermal processing must be performed in an environment that is relatively free of contamination. For this reason, the processing chamber is pressurized to protect against contamination from the ambient air in the manufacturing environment. The orifice through which a quartz elevator tube protrudes to support the wafer includes a bearing whose surface co acts with the elevator tube to define a gas curtain made up of chamber gas escaping the chamber and pressurized gas fed to the bearing. The gas curtain also reduces the lateral motion of the elevator tube as it moves vertically within the chamber. To minimize contamination, it is desirable to maintain low clearance between the bearing surface in the chamber orifice and the elevator tube. Another advantage of lower clearance is that it enhances the centering capability of the gas curtain by increasing the pressure drop along the tube axis for a given flow rate of gas within the gas curtain. However, low clearance between the bearing and the elevator tube increases the possibility that the elevator tube will contact the bearing and pick up contaminants from the bearing that may be carried into the processing chamber.